1). Field of the Invention
This invention relates generally to the structure and fabrication of resistors in an integrated circuit and more particularly to resistors in a thermal ink jet printing head.
2). Description of the Prior Art
Ink jet printing systems can be divided into two basic types. One type uses a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle. The other type uses thermal energy to produce a vapor bubble in an ink filled channel that expels a droplet. This latter type is referred to as thermal ink jet printing or bubble jet printing. Generally, thermal ink jet printing systems have a print head comprising one or more ink filled channels that communicate with a relatively small ink supply chamber at one end, and have an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in the channels near the nozzle at a predetermined distance upstream therefrom. The resistors are individually addressed with a current pulse representative of data signals to momentarily vaporize the ink and formed a bubble which expels an ink droplet. FIG. 1 shows an electrical schematic of one ink jet of a printhead having a resistor 100 and a power transistor 102. In fabrication, the ink supply chamber is located over the resistor and the power transistor is formed nearby on a substrate. One preferred method of fabricating thermal ink jet printheads is to form the heating elements on the surface of one silicon wafer and the channels and small ink supply chamber of reservoir on the surface of another silicon wafer.
In many integrated circuit applications, especially ink jet printheads, there is a need for structures which function as resistors. For years, widely doped silicon stripes have been used as resistors for a wide variety of applications. Most semiconductor manufacturers have abandoned this particular use of polysilicon resistors for several reasons. One reason is junction spiking. Not only is the resistivity of the polysilicon non-linear with respect to voltage, but it is difficult to achieve resistive values consistently in such structures due to three variables: deposit related polysilicon film thickness, etch dependent film width, and uniform doping levels. The three variables interact to establish the resistive value of the structure (resistor). Because the variability is too great, many manufacturers utilize a metal layer or a combination polysilicon and metal to create a mult-level resistor structures.
A major problem in the manufacture of thermal ink jet printhead is the resistor and power transistor quality and yields. FIG. 1 shows a resistor 100 connected to a power transistor 102. The resistor must be made of a material that has a controllable resistivity.
Many practitioners have improved the resistors and printheads. The most pertinent are as follows: U.S. Pat. No. 4,789,425 (Drake), U.S. Pat. No. 5,384,442 (Danner), U.S. Pat. No. 5,429,554 (Tunura), U.S. Pat. No. 5,387,314 (Baughman et al.) and U.S. Pat. No. 5,368,683 (Altavela) show the FAB methods and resulting structures of ink filled head with heater resistor. U.S. Pat. No. 5,496,762 (Sandhu) shows the use of a TiNC resistor. U.S. Pat. No. 5,420,063 (Mayhsoudnia) used a resistor layer of SiCr, NICr, TaN, CiCR plus a conductive layer of TiN as a resistive layer. However, printheads and resistors can be further improved to make them more reliable, especially at higher temperatures and less complicated to manufacture.